Microcells, which are usually small, low-power radio base stations having small coverage areas, may be used to provide wireless communications in localized areas. Microcells may be coupled to larger base stations, or macrocells, through digital radio transmission or optical fibers. A typical cell site, either a microcell or a macrocell, includes one or more transceivers and a transmitter such as an antenna, and supplies communication services to subscribers within a designated coverage area of the cell site.
Designing a communication system which includes both microcells and macrocells requires selecting geographic locations of transmitters, their desired coverage areas and their signal radiation patterns, so that specific frequencies, such as radio frequencies (RFs), are present within each cell's designated coverage area, while interference with other coverage areas sharing the same or similar frequencies is minimized.
Prediction of actual signal propagation patterns into and out of cells may be difficult, however, because buildings and other terrain irregularities may reflect signals in unanticipated directions, resulting in insufficient signal strength in a one cell's designated coverage area and/or unacceptable levels of interference in another cell's coverage area.
Several software tools are available which predict signal propagation paths and aid in planning the physical layout of a wireless communication system. Examples of such tools include, but are not limited to, Motorola's Netplan.TM. RF system planning and management tool, Mobile Systems, Inc.'s PlaNet.TM. RF coverage tool, LNS's RF planning tool and a commercially available raytracing tool developed by T. Rappaport. Currently, however, these and other software tools may not be suitable for use in planning all types of communication systems.
First, environmental models or high-resolution maps which model, typically via information stored in a database, areas of interest within a geographic area associated with the communication system and are used to predict signal characteristics at a particular location are different for macrocells than for microcells. For example, macrocellular maps typically require less resolution (or variable layers of resolution) than microcellular maps, which typically are modeled using high resolution databases.
Second, simplistic pathloss equations and empirical, or statistical, propagation models, are generally used in connection with predicting signal characteristics, such as signal strength, associated with macrocells; while for microcells more sophisticated ray tracing techniques, or deterministic models, are used to predict signal characteristics such as actual signal propagation paths. In general, databases associated with macrocellular propagation models are in different coordinate systems and/or formats than databases associated with microcellular propagation models.
Although possible, it may be time consuming and expensive to manually select and field test different combinations of frequencies, signal radiation patterns and/or signal powers to obtain complete coverage and acceptable interference levels for each cell in a communication system which includes both microcells and macrocells.
There is therefore a need for a method and apparatus for predicting signal characteristics in a communication system having both microcells and macrocells which integrates maps having different resolutions into a single map having a single coordinate system, and which adaptively adjusts the prediction of signal characteristics based on the location of a particular point of interest.